WASHINGTON (CNN) -- The morning fog shrouding Arlington National Cemetery creates a mystical atmosphere around the graves of American servicemen and women buried there with military honors.

Tourists gather at the Tomb of the Unknowns, on a hill overlooking the nation's capital, to watch one of the simplest of ceremonies: the changing of the guard.

Most of the visitors have no direct connection to the wars in which the unknowns served. Even Vietnam seems a distant memory to many who come to pay their respects.

Still, they watch in silence as the armed Army sentries march with agonizingly slow cadence along the row of marble monuments where three unidentified soldiers are entombed in crypts, one each from World War II and the Korean and Vietnam wars. (The remains of a fourth unknown, from World War I, are interred behind the row.)

This week, the silence was broken by another sound. In the still hours of the night, a drill pierced the marble to exhume the remains of an American killed in Vietnam.

That's an extraordinary act: opening the Tomb of the Unknowns because the Pentagon now has second thoughts about whether one of its occupants really is "unknown." Five families hold hopes the remains could be those of their loved ones.

Since the remains were interred in 1984, the family of Air Force 1st Lt. Michael Blassie, a pilot whose plane was shot down in Vietnam in 1972, has come to believe they are his.

That conclusion, based on the revelation of other evidence the government had, led to a decision to exhume the remains for DNA testing that was not possible when the remains were recovered from Blassie's crash site in An Loc, Vietnam.

Like the fog that contributes to the mysticism on a spring morning at the cemetery, there is an emotional atmosphere that still surrounds the politics of the Vietnam War.

The exhumation of the remains in the Tomb of the Unknowns and the new identification testing represent an important transition in these politics.

Ever since the world saw pictures of the U.S. military pulling out of Saigon in 1974, with South Vietnamese refugees clinging to the helicopters, the American people have struggled with mixed success to put that war's agony behind them.

Prisoners and soldiers came home to little fanfare. Many Vietnam veterans still feel they were slighted by their country for their sacrifice in the battle against communism in southeast Asia.

It was only last year -- some 25 years after the war -- that the United States sent an ambassador back to Vietnam.

And reminders remain. Every day on his way to work in Hanoi, U.S. Ambassador Pete Peterson passes the place he was forced to call home when his plane was shot down during the war -- the Maison Centrale, the high-security prison that captive Americans called "The Hanoi Hilton."

U.S. economic and political sanctions against Vietnam have been eased only recently. And the Vietnamese ambassador to Washington still is jeered in the United States by former South Vietnamese refugees who have become American citizens since they fled from the Hanoi government's takeover of the south.

Much of the rest of the world has moved on with Vietnam. Asian and European investors are taking advantage of an industrious Vietnamese population.

Amid the gradual transition in thinking about Vietnam in the
United States, Americans are testing the waters for business there, too. A mammoth Ford Motor Company factory looms among the rice fields between Hanoi and Haiphong, two cities the United States bombed mercilessly during the war.

The U.S. economic stake in Asia has dramatically increased since the Vietnam War era. The current Asian financial crisis notwithstanding, this involvement in the region is only likely to grow.

For many Americans, the intensity of the unpleasant memories of the Vietnam War is fading.

But the memories are not gone. That's why the Tomb of the Unknowns was opened.

And as cases such as those of Blassie are resolved with delicate ceremony and modern science, the United States is slowly turning a page of history toward a new relationship with southeast Asia.